1. Field of the Invention
The invention relates generally to amplifier designs for solid-state lasers and more particularly to amplifier designs for transversely-pumped, diode-pumped solid-state lasers.
2. The Prior Art
Conventional optically-pumped solid-state lasers utilize broadband arc lamps or flashlamps to laterally or transversely pump the solid-state laser medium in a resonant cavity. The direction of pumping is transverse or orthogonal to the longitudinal axis of the resonant cavity. The entire medium is pumped so there is little correspondence between the pump volume and the TEMOO mode volume defined by the laser cavity; operation in TEMOO mode is desired. Much of the pumping energy goes into regions of the medium outside the volume occupied by the laser mode and therefore does not contribute to amplification of the laser beam. Thus pumping efficiency is low (typically a few percent).
Laser diodes form efficient pumping sources; a variety of different types of laser diodes, particularly laser diode arrays, e.g. Spectra Diode Labs Model No. 2410 GaAlAs laser diode array, in which a plurality of emitters are phase locked together, and extended emitter laser diodes, e.g. Sony Model Nos. SLD 301, 302, 303, 304 V/W, have been or can be used. U.S. Pat. Nos. 4,653,056 and 4,656,635 and patent application Serial No. 029,836 filed Mar. 24, 1987 and patent application Ser. No. 035,530, filed Apr. 7, 1987 describe a solid-state laser longitudinally end pumped by a laser diode source in which the pump volume is matched to the desired TEMOO mode volume to optimize pumping efficiency. In the longitudinal end pump configuration, the direction of pumping coincides with the longitudinal axis of the resonator cavity, and thus can be matched into the laser mode volume. U.S. Pat. No. 4,665,529, issued May 12, 1987 and patent application Ser. No. 048,717 filed May 12, 1987 describe a solid-state laser in which a laser diode source is coupled to a laser rod by means of an optical fiber to longitudinally end pump and mode match the laser. It is desirable to produce small size, low cost, high performance solid-state lasers.
Thus the resonator/pump configuration is a key feature of laser design and performance. Lateral pumping schemes do not provide mode matching and are therefore inefficient. End pumping schemes using laser diodes provide mode matching and consequently high efficiency. However, previously-available laser diodes have often been limited in power, usually under 1 W. Furthermore, even with higher power laser diode sources, the end pumped configuration limits the amount of energy that can be used, thereby limiting the power of the laser, since the power densities in the pump region of the gain medium become too high and the heat produced cannot be removed. This heat is disadvantageous in that it causes thermal focusing and bulk heating leading to general inefficiency. Accordingly, it is desirable to provide a resonator configuration which combines a transverse or lateral pump geometry with mode matching of the pump volume to the TEMOO mode volume since lateral pumping allows more energy to be input into the medium while mode matching uses the pump energy more effectively.
Another type of laser diode is a plurality of laser diode arrays fabricated into a multi-element bar structure. These laser diode array bars typically have ten 1 W laser diode arrays spaced along a 1 cm bar; each array has multiple emitters phase locked together. These array bars are not suitable for end pumping a solid-state laser but could be useful for transversely or laterally pumping a solid-state laser. However, if the bars are used as mere substitutes for arc lamps, little benefit will be derived. Accordingly it is necessary to develop a laser amplifier/pump configuration in which the output of the laser diode array bar can be mode-matched to a desired mode volume (TEMOO) within the solid-state laser material.
It is sometimes desireable to obtain laser power outputs greater than 10 watts. However, at these power levels, at least two problems occur which prevent realization of larger power outputs. First, pump power must be increased. As the pump power increases, the laser rod heats up, thus reducing laser efficiency. Cooling of the rod is difficult.
Secondly, and as a result of the heating, thermal gradients form in the laser medium, giving rise to the thermal focusing phenomenon resulting from differing indices of refraction in adjacent portions of the crystal at differing temperatures. This thermal focusing can result in damage to the laser medium. Thus it is always desireable to limit the thermal focal lengths in the laser medium to values much larger then the optical path through the medium, to avoid damage to the crystal structure. It is therefore desired to provide a laser having a high power output which avoids these problems.
Optical amplifiers provide a useful means of increasing the power level of a laser beam. These devices have found applications in tunable dye laser amplification, lightwave communication, laser fusion, and many other laser systems. Several different types of optical amplifiers currently exist. Most of these amplifiers are pulsed since it is easier in pulsed systems to obtain the levels of population inversion necessary for large gain.
Recently, continuous wave optical amplifiers have been designed based primarily on rare earth doped fibers. These designs utilize diffraction-limited laser diodes as pump sources. These diodes are coupled into a single mode fiber doped with Nd or Er. The pump light is guided along the fiber and excites a long narrow column of rare earth ions. The signal wave (at the lasing wavelength of the rare earth ion) is also injected into the fiber and is guided along the same path as the pump. The signal wave is amplified by stimulated emission from the rare earth ions. There are several limitations to fiber laser amplifiers. First they require that the pump beam be near diffraction limited to couple into the single mode fiber. This limits the amount of power that can be effectively used to pump the amplifier. The limited amount of pump restrict the total amount of power that one can extract from the amplifier. Second, the range of wavelengths that can be amplified is limited to those ions and transitions that are compatible with fiber laser manufacturing techniques. Third, in order to extract high power from the amplifier a large population inversion is necessary. The large population inversion gives rise to very high small signal gains which leads to amplified spontaneous emission and lasing off of spurious reflections. Fourth, the lasing transition in glass fibers is primarily imhomogenously broadened. Thus it is difficult or impossible for a narrow bandwidth signal beam to efficiently saturate the active medium. This leads to significant undepleted gain in the fiber amplifier causing excess noise and the potential for lasing off of spurious reflections.
Thus alternative designs are necessary to obtain optical amplifiers that permit the extraction of high power and allow high gain. It is important that the lasing transitions of the amplifier be homogeneously broadened so as to allow uniform saturation of the excited active medium across the laser gain profile. The amplifier should be configured to allow the flexibility to adjust the pump volume region so as to achieve useful gain (10 dB) and yet allow the extraction of high average powers (hundreds of milliwatts to watts).
In order to extract high power, the laser diode pump power must be efficiently coupled into the volume area occupied be the incident signal beam. Moreover the size of the pump beam must be adjusted to provide sufficiently high gain along the path of the signal beam. In general this means minimizing the pump volume along this path. It is also important that parasitic oscillations in the amplifier be eliminated by reducing or eliminating feed back along optical paths within the amplifier. Finally it is important to design the pumping scheme of the amplifier to minimize optical distortion of the signal beam as it propagates through the amplifier.